Modeling positions of implanted devices in a patient

ABSTRACT

Technology is disclosed for modeling positions of implanted devices in a patient. In various embodiments, the technology can construct a forward model that predicts an electrical impedance between electrical contacts; detects an actual electrical impedance between electrical contacts; computes a fitness value based on a comparison between the detected electrical impedance and the predicted electrical impedance; varies at least one parameter of the forward model until the computed fitness value is a maximum fitness value; and displays at a display device a estimated position of the first lead and/or second leads.

CROSS-REFERENCE TO RELATED APPLICATION

This application divisional application to U.S. patent application Ser.No. 13/645,387, filed Oct. 4, 2012, which claims the benefit to U.S.Provisional Patent Application No. 61/543,257, filed Oct. 4, 2011,entitled “MODELING POSITIONS OF IMPLANTED DEVICES IN A PATIENT,” andU.S. Provisional Patent Application No. 61/543,766, filed Oct. 5, 2011,entitled “MODELING POSITIONS OF IMPLANTED DEVICES IN A PATIENT,” each ofwhich is incorporated herein by reference in its entirety.

This application is related to U.S. patent application Ser. No.12/895,403 (attorney docket no. 66245-8025.US00), entitled “SYSTEMS ANDMETHODS FOR POSITIONING IMPLANTED DEVICES IN A PATIENT”, and filed onSep. 30, 2010, which application is incorporated herein by reference inits entirety.

The following patent applications are incorporated herein by referencein their entireties: U.S. application Ser. No. 12/468,688, filed May 19,2009 and titled IMPLANTABLE NEURAL STIMULATION ELECTRODE ASSEMBLIES ANDMETHODS FOR STIMULATING SPINAL NEURAL SITES U.S. application Ser. No.12/362,244, filed Jan. 29, 2009 and titled SYSTEMS AND METHODS FORPRODUCING ASYNCHRONOUS NEURAL RESPONSES TO TREAT PAIN AND/OR OTHERPATIENT CONDITIONS; U.S. application Ser. No. 12/499,769, filed Jul. 8,2009 and titled SYSTEMS AND METHODS FOR ADJUSTING ELECTRICAL THERAPYBASED ON IMPEDANCE CHANGES; U.S. application Ser. No. 12/765,685, filedApr. 22, 2010 and titled SPINAL CORD MODULATION FOR INDUCING PARESTHETICAND ANESTHETIC EFFECTS, AND ASSOCIATED SYSTEMS AND METHODS; U.S.application Ser. No. 12/765,747, filed Apr. 22, 2010 and titledSELECTIVE HIGH FREQUENCY SPINAL CORD MODULATION FOR INHIBITING PAIN WITHREDUCED SIDE EFFECTS, AND ASSOCIATED SYSTEMS AND METHODS; U.S.application Ser. No. 12/765,790, filed Apr. 22, 2010 and titled DEVICESFOR CONTROLLING HIGH FREQUENCY SPINAL CORD MODULATION FOR INHIBITINGPAIN, AND ASSOCIATED SYSTEMS AND METHODS, INCLUDING SIMPLIFIEDCONTROLLERS; U.S. Provisional Application No. 61/418,379, filed Nov. 30,2010 and titled EXTENDED PAIN RELIEF VIA HIGH FREQUENCY SPINAL CORDMODULATION, AND ASSOCIATED SYSTEMS AND METHODS; U.S. application Ser.No. 12/264,836, filed Nov. 4, 2008 and titled MULTI-FREQUENCY NEURALTREATMENTS AND ASSOCIATED SYSTEMS AND METHODS; and U.S. application Ser.No. 13/607,617, filed Sep. 7, 2012 and titled SELECTIVE HIGH FREQUENCYSPINAL CORD MODULATION FOR INHIBITING PAIN, INCLUDING CEPHALIC AND/ORTOTAL BODY PAIN WITH REDUCED SIDE EFFECTS, AND ASSOCIATED SYSTEMS ANDMETHODS.

TECHNICAL FIELD

The disclosed technology is directed generally to modeling positions ofimplanted devices in a patient.

BACKGROUND

Neurological stimulators have been developed to treat pain, movementdisorders, functional disorders, spasticity, cancer, cardiac disorders,and various other medical conditions. Implantable neurologicalstimulation systems generally have an implantable pulse generator andone or more leads that deliver electrical pulses to neurological tissueor muscle tissue. For example, several neurological stimulation systemsfor spinal cord stimulation (SCS) have cylindrical leads that include alead body with a circular cross-sectional shape and multiple conductiverings spaced apart from each other at the distal end of the lead body.The conductive rings operate as individual electrodes and the SCS leadsare typically implanted by practitioners either surgically orpercutaneously through a large needle inserted into the epidural space,with or without the assistance of a stylet.

Once implanted, the pulse generator applies electrical pulses to theelectrodes, which in turn modify the function of the patient's nervoussystem, such as by altering the patient's responsiveness to sensorystimuli and/or altering the patient's motor-circuit output. During paintreatment, the pulse generator applies electrical pulses to theelectrodes, which in turn can generate sensations that mask or otherwisealter the patient's sensation of pain. For example, in many cases,patients report a tingling or paresthesia that is perceived as morepleasant and/or less uncomfortable than the underlying pain sensation.In other cases, the patients can report pain relief without paresthesiaor other sensations.

In any of the foregoing neurological stimulation systems, it isimportant for the practitioners to accurately position the stimulator toprovide effective therapy. One approach to accurately positioning thestimulator is to implant the stimulator in a surgical procedure so thatthe practitioner has a clear visual access to the implantation site.However, many patients and practitioners wish to avoid the invasivenessand associated likelihood for complications typical of a surgicalprocedure. Accordingly, many patients and practitioners prefer a lessinvasive (e.g., percutaneous) implantation technique. With apercutaneous approach, the practitioner typically is unable to seeexactly where the device is positioned because the device is beneath thepatient's skin and, in most SCS cases, is within the patient's spinalcolumn. In addition, the process typically requires the patient toprovide feedback to the practitioner based on that patient's sensations.Accordingly, the industry has developed a variety of techniques forvisualizing medical devices and anatomical features below the patient'sskin. Such techniques include fluoroscopy, which is commonly used to aidthe practitioner when implanting SCS leads. However, a drawback withfluoroscopy is that it results in added expense to the SCS implantationprocedure, it may be cumbersome to implement, it limits the implantationprocedure to sites with fluoroscopy equipment, and it exposes thepatient to unwanted x-ray radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a partially schematic illustration of an implantable spinalcord modulation system positioned at a patient's spine to delivertherapeutic signals in accordance with several embodiments of thepresent disclosure.

FIG. 1B is a partially schematic, cross-sectional illustration of apatient's spine, illustrating representative locations for an implantedlead in accordance with an embodiment of the disclosure.

FIG. 2 is a partially schematic illustration of a representative signaldelivery device, signal transmission device, and signal detectiondevice, configured in accordance with various embodiments of thedisclosure.

FIG. 3 is a graph diagram illustrating Cartesian coordinates that can beused in various embodiments of the disclosed technology to identify aposition of a second lead relative to a first lead.

FIG. 4 is a flow diagram illustrating a routine that the disclosedtechnology may invoke in various embodiments, e.g., to identify aposition of a second lead relative to a first lead.

FIG. 5 is a block diagram illustrating components employed by thedisclosed technology in various embodiments.

FIG. 6 is a user interface diagram illustrating a user interfaceimplemented by the disclosed technology in various embodiments.

DETAILED DESCRIPTION

The disclosed technology is directed generally to modeling positions ofimplanted devices in a patient. In various embodiments, the technologyis used to assist a practitioner implanting leads proximate to apatient's spinal cord to deliver high frequency signals that modulateneural activity at the patient's spine, e.g., to address chronic pain.In other embodiments, however, the systems and associated methods canhave different configurations, components, procedures and/or purposes.Still other embodiments may eliminate particular components orprocedures. A person of ordinary skill in the relevant art, therefore,will understand that the technology may include other embodiments withadditional elements, and/or may include other embodiments withoutseveral of the features shown and described below with reference to theFigures. The technology estimates the relative positions of leadspositioned in the fatty epidural space dorsal to the spinal cord andwithin the spine of the patient. By modeling impedances between multiplecontacts (e.g., electrical contacts) on each lead, computing a fitnessvalue to represent similarity between an output of the model andmeasured impedances, and attempting to maximize the fitness value, thetechnology can accurately determine the relative positions of leads.

In various embodiments, an implanted stimulation system includes twoleads and each lead may have eight contacts. By modulating electricalsignals at the leads and/or contacts, the implanted stimulation systemcan reduce pain that a patient experiences, e.g., in the patient'sspinal cord area. In other embodiments, more or fewer leads and/orcontacts may be employed. In various embodiments, the technology candetect impedances between contacts, e.g., between any two specifiedcontacts. In other embodiments, the technology can detect impedances ofpaths between any two poles. A pole is a set of one or more contactsconnected electrically and functioning as a unit. In these embodiments,the casing or other electrically conductive components of thestimulation system may be used as contacts. The technology can assumethat impedance along a given path is a function of a three-dimensionalCartesian length of the path, e.g., a sum of the impedance of the paththrough tissue plus one or more impedance values characteristic to theelectrode-tissue interface at the contacts involved in the path. Inother embodiments, other models of impedance may be employed. Thetechnology constructs a model (e.g., a “forward model”) of the expectedimpedance measured between contacts of the two leads, e.g., betweencontact of a first lead and each contact of a second lead. The model cantake as input various parameters, e.g., geometric parameters such asdistances between the two leads at various points, rotation about axes,etc; various physical parameters such as resistivity or specificimpedance of nearby tissue; and parameters representing impedance valuesat each contact. The impedance value for each contact is the impedanceof an electrode-tissue interface at that contact. The model can thenoutput a set of predicted impedance measurements. The technology canthen compute a fitness value that compares the output of the model withactually detected impedance values. In other embodiments, the technologycan compute a distance value that expresses the difference between theoutput of the model and the actually detected impedance values. Byemploying techniques for varying the inputs, producing outputs, andmaximizing a computed fitness value or minimizing a computed distancevalue, the technology can accurately detect the relative position ofleads. Values for some or all parameters (e.g., geometric parameters)representing the detected relative position can be drawn from acontinuous range of values, e.g., a range of possible values that arenot limited to discrete or predetermined values. Thus, the technologyemploys the forward model to compute the position of leads withoutrequiring predetermined positions for the leads and without requiringprecalculated impedance measurement predictions for these predeterminedpositions.

In various embodiments, the forward model can be used to optimize theposition of the leads. As an example, a practitioner may attempt tolocate an optimal position for the leads, e.g., to minimize patientdiscomfort. The technology can employ the forward model to predict therelative position of leads, e.g., at some point in the future. As anexample, by analyzing the vectors of movement of the leads, thetechnology can guide the practitioner, e.g., by displaying on a screenthe predicted positions of the leads. The technology may determine thispredicted position by assuming a constant direction and rate of movementand calculating the expected position if the direction and rate were toremain constant. As another example, a prediction of lead position basedon previous lead movement data may be used to provide constraints,starting values, or historical comparisons for estimates of current leadposition using the technology described herein. This previous leadmovement data may include lead positions estimated from impedance orother measurements, lead positions directly measured using fluoroscopyor other techniques, or a combination of these data sources.

Several embodiments of the technology are described in more detail inreference to the Figures. The computing devices on which the describedtechnology may be implemented may include one or more central processingunits, memory, input devices (e.g., keyboard and pointing devices),output devices (e.g., display devices), storage devices (e.g., memory,disk drives, etc.), and network devices (e.g., network interfaces). Thememory and storage devices are computer-readable media that may storeinstructions that implement aspects of the disclosed technology. Thesememory and storage devices may be tangible and/or non-transitory invarious embodiments. In addition, the data structures and messagestructures may be stored or transmitted via a data transmission medium,such as a signal on a communications link. Various communications linksmay be used for computing devices to communicate, such as the Internet,a local area network, a wide area network, or a point-to-point dial-upconnection.

FIG. 1A schematically illustrates a representative patient system 100for providing relief from chronic pain and/or other conditions, arrangedrelative to the general anatomy of a patient's spinal cord 191. Theoverall patient system 100 can include a signal delivery system 110,which may be implanted within a patient 190, typically at or near thepatient's midline 189, and coupled to a pulse generator/receiver 121.The signal delivery system 110 can provide therapeutic electricalsignals to the patient during operation. The overall patient system 100can further include a signal transmission system 130. The signalshandled by the signal transmission system 130 can function to identifythe location of the signal delivery system 110 and/or deliver relieffrom pain. Accordingly, the signal transmission system 130 can operateindependently of the signal delivery system 110 to guide thepractitioner as he/she positions elements of the signal delivery system110 within the patient. Nevertheless, in particular embodiments, certainelements of the signal transmission system 130 can be shared with thesignal delivery system 110. Aspects of the signal delivery system 110are described immediately below. In some embodiments, the signaltransmission system is absent.

In various embodiments, the signal delivery system 110 includes a signaldelivery device 111 that includes features for delivering therapy to thepatient 190 after implantation. The pulse generator/receiver 121 can beconnected directly to the signal delivery device 111, or it can becoupled to the signal delivery device 111 via a signal link 113 (e.g.,an extension). In various embodiments, the signal delivery device 111can include elongated leads or lead bodies 112 and 212. As used herein,the terms “lead” and “lead body” include any of a number of suitablesubstrates and/or support members that carry devices for providingtherapy signals to the patient 190. For example, the lead 112 caninclude one or more electrical contacts (e.g., electrodes) that directelectrical signals into the patient's tissue, such as to provide forpatient relief. In other embodiments, the signal delivery device 111 caninclude structures other than a lead body (e.g., a paddle) that alsodirect electrical signals and/or other types of signals to the patient190. In various embodiments, lead 112 delivers signals and lead 212detects the delivered signals. In other embodiments, both leads 112 and212 deliver signals.

The pulse generator/receiver 121 can transmit signals (e.g., electricalsignals) to the signal delivery device 111 that up-regulate (e.g.,stimulate or excite) and/or down-regulate (e.g., block or suppress)target nerves. As used herein, and unless otherwise noted, the terms“modulate” and “modulation” refer generally to signals that have eithertype of the foregoing effects on the target nerves. The pulsegenerator/receiver 121 can include a computing device with instructionsfor generating and transmitting suitable therapy signals. The pulsegenerator/receiver 121 and/or other elements of the system 100 caninclude one or more processors 122, memories or other storage devices123 and/or input/output devices. Accordingly, the process of providingmodulation signals, providing guidance information for locating thesignal delivery device 111, and/or executing other associated functionscan be performed by computer-executable instructions stored incomputer-readable media or storage devices (e.g., memory) located at thepulse generator/receiver 121 and/or other system components. The pulsegenerator/receiver 121 can include multiple portions, elements, and/orsubsystems (e.g., for directing signals in accordance with multiplesignal delivery parameters), carried in a single housing, as shown inFIG. 1A, or in multiple housings.

In some embodiments, the pulse generator/receiver 121 can obtain powerto generate the therapy signals from an external power source 118. Theexternal power source 118 can transmit power to the implanted pulsegenerator/receiver 121 using electromagnetic induction (e.g., RFsignals). For example, the external power source 118 can include anexternal coil 119 that communicates with a corresponding internal coil(not shown) within the implantable pulse generator/receiver 121. Theexternal power source 118 can be portable for ease of use.

During at least some procedures, an external programmer 120 (e.g., atrial modulator) can be coupled to the signal delivery device 111 duringan initial procedure, prior to implanting the pulse generator/receiver121. For example, a practitioner (e.g., a physician, a technician,and/or a company representative) can use the external programmer 120 tovary the modulation parameters provided to the signal delivery device111 in real time, and select optimal or particularly efficaciousparameters. These parameters can include the location from which theelectrical signals are emitted, as well as the characteristics of theelectrical signals provided to the signal delivery device 111. In atypical process, the practitioner uses a cable assembly 114 totemporarily connect the external programmer 120 to the signal deliverydevice 111. The practitioner can test the efficacy of the signaldelivery device 111 in an initial position (e.g., a specified position,which may be ±5 mm from a particular point on or near the patient'sspine). The practitioner can then disconnect the cable assembly 114(e.g., at a connector 117), reposition the signal delivery device 111,and reapply the electrical modulation. This process can be performediteratively until the practitioner obtains the desired position for thesignal delivery device 111. Optionally, the practitioner may move thepartially implanted signal delivery device 111 without disconnecting thecable assembly 114.

After a trial period with the external programmer 120, the practitionercan implant the implantable pulse generator/receiver 121 within thepatient 190 for longer term treatment. The signal delivery parametersprovided by the pulse generator/receiver 121 can still be updated afterthe pulse generator/receiver 121 is implanted, via a wirelessphysician's programmer 125 (e.g., a physician's remote) and/or awireless patient programmer 124 (e.g., a patient remote). The patient190 may have control over fewer parameters than does the practitioner.

FIG. 1B is a cross-sectional illustration of the spinal cord 191 and anadjacent vertebra 195 (based generally on information from Crossman andNeary, “Neuroanatomy,” 1995 (published by Churchill Livingstone)), alongwith multiple signal delivery devices 111 (shown as signal deliverydevices 111 a-d) implanted at representative locations. For purposes ofillustration, multiple signal delivery devices 111 are shown in FIG. 1Bimplanted in a single patient. In actual use, any given patient willlikely receive fewer than all the signal delivery devices 111 shown inFIG. 1B. As an example, a patient may receive two leads in one or twosignal delivery devices 111.

The spinal cord 191 is situated within a vertebral foramen 188, betweena ventrally located ventral body 196 and a dorsally located transverseprocess 198 and spinous process 197. Arrows V and D identify the ventraland dorsal directions, respectively. The spinal cord 191 itself islocated within the dura mater 199, which also surrounds portions of thenerves exiting the spinal cord 191, including the ventral roots 192,dorsal roots 193 and dorsal root ganglia 194. In one embodiment, asingle first signal delivery device 111 a is positioned within thevertebral foramen 188, at or approximately at the spinal cord midline189. In another embodiment, two second signal delivery devices 111 b arepositioned just off the spinal cord midline 189 (e.g., about 1 mm.offset) in opposing lateral directions so that the two signal deliverydevices 111 b are spaced apart from each other by about 2 mm. In stillfurther embodiments, a single signal delivery device or pairs of signaldelivery devices can be positioned at other locations, e.g., at thedorsal root entry zone as shown by a third signal delivery device 111 c,or at the dorsal root ganglia 194, as shown by a fourth signal deliverydevice 111 d.

In any of the foregoing embodiments, it may be useful that the signaldelivery device 111 be placed at a target location that is expected(e.g., by a practitioner) or indicated (e.g., by the patient) to produceefficacious results in the patient when activated. The followingdisclosure describes techniques and systems for improving the level ofaccuracy with which the devices are positioned.

FIG. 2 is a partially schematic illustration of a representative signaldelivery device, signal transmission device, and signal detectiondevice, configured in accordance with various embodiments of thedisclosure. A representative signal delivery device 111 includes leads112 and 212, each carrying a plurality of ring-shaped therapy contacts.Each contact may transmit and/or receive signals from one or moreelectrodes. Contacts 126 a-h of lead 112 are positioned toward a distalend of lead 112 to deliver a therapy signal to the patient when the lead112 is implanted. The lead 112 may include internal wires and/or one ormore electrodes (not visible in FIG. 2). During implantation, animplanting tool 160 (e.g., a stylet 161) is temporarily coupled to thelead 112 to support the lead 112 as it is inserted into the patient. Forexample, the implanting tool 160 can include a shaft 162 that isslideably and releasably inserted (via, e.g., a handle 163) into anaxially-extending opening in the lead 112. The shaft 162 is generallyflexible, but more rigid than the lead 112 to allow the practitioner toinsert the lead 112 and control its position during implantation. Astylet stop 128 at the distal end of the lead opening prevents thepractitioner from over-inserting the stylet shaft 162. Lead 212 issimilar to lead 112. In some embodiments, lead 212 includes a pluralityof contacts 226 a-h to deliver and/or receive signals. Lead 212 may beadjustable separately from lead 112, e.g., by use of handle 263. Invarious embodiments, leads 112 and 212 each have eight contacts. In someembodiments, each lead may have a single electrode with multiplecontacts or multiple contacts for each of multiple electrodes.

The lead 112 and/or other portions of the overall system 100 can includefeatures that guide the practitioner when positioning the lead 112 at atarget location. For example, the signal transmission system 130 can becarried by the lead 112 and/or the implanting tool 160, and cancommunicate with the signal detector systems located outside thepatient's body. Signal transmission devices 131 a and 131 b cangenerate, emit, and/or reflect the locator signals 132 a-h in a mannerthat is detected by a signal detector device of a signal detectorsystem. Although eight locator signals 132 a-h are illustrated, theremay be more or fewer locator signals (even in embodiments with eightcontacts per lead). In some embodiments, the signal detector system islead 212. In other embodiments (not illustrated), the signal detectorsystem may be a system external to the patient. The first signaltransmission device 131 a can be carried by the lead 112, and can beindependent of (e.g., electrically isolated from) the therapy contacts126. The second signal transmission device 131 b can also be carried bythe lead 112, but can double as one of the therapy contacts 126. In someembodiments, the second signal transmission device 131 b doubles as thedistal-most therapy contact 126, located at or near the distal tip ofthe lead 112. In other embodiments, the second signal transmissiondevice 131 b can double as any of the other therapy contacts 126. Thethird signal transmission device 131 c is carried by the implanting tool160, rather than the lead 112. For example, the third signaltransmission device 131 c can be located at the distal-most tip of theimplanting tool 160.

A patient may respond to treatment differently depending on the relativepositions of leads 112 and 212. For example, a patient may experiencesome relief from pain when lead 212 is at a first position relative tolead 112 and additional relief from pain when lead 212 is at a secondposition relative to lead 112. The position of a first lead may bedetermined, e.g., by using technology disclosed in the relatedapplication identified in the first paragraph of this patentspecification. The relative position of the second lead (e.g., incomparison to the position of the first lead) can be specified usingthree-dimensional Cartesian coordinates.

The relative position of the second lead as determined by the disclosedtechnology may also be used to identify leads in a fluoroscopic or otherimage. Such other images may be ambiguous or difficult to read due tooverlap of leads in the image and projection of complex lead geometryinto an image plane. In such a case, the disclosed technology maydetermine that one lead is higher in the epidural canal, therebyenabling unambiguous identification if the image similarly indicatesthat one lead is positioned higher than the other.

In some embodiments, impedance measurements usable with the disclosedtechnology may be collected by the implantable pulse generator/receiver121 or triggered by the wireless physician's programmer 125 or thewireless patient programmer 124. These measurements may be collectedregularly, e.g. periodically, or on a schedule determined by physicianor patient activity. A series of relative lead positions over time maythen be estimated using the methods disclosed herein, correlated to painor other clinical data indicative of device efficacy, used to predict orconstrain future estimates of lead position, and/or displayed to thepractitioner.

FIG. 3 illustrates a forward model 300 that can be used in variousembodiments of the disclosed technology, including coordinates used, toidentify a position of a second lead relative to a first lead. Takingone end of lead 112 (or a contact of lead 112) as an origin in thecoordinate system 300, a parameter z 302 can indicate a z-coordinate ofa corresponding end of lead 212 (or a contact of lead 212). A parameterr 301 can indicate a radial distance (e.g., in cylindrical coordinates)between a specified contact on each lead, e.g., a least distal contactfrom a first end of each lead. Alternatively, parameter r 301 canindicate a radial distance between an axis (e.g., a z-axis) and thecorresponding end of lead 212 (or a contact of lead 212). Theorientation of lead 212 may also be compared relative to lead 112 in oneor more rotational axes. As an example, θ 304 and Φ 303 can indicate, inspherical coordinates, an inclination of lead 212 measured from a zenithlocated on the z-axis and an azimuth of lead 212 measuredcounter-clockwise from a plane formed by lead 112 and a contact (e.g., aleast distal contact from a first end of lead 212). By using the valuesr, z, φ, and θ, the position of the second lead relative to the positionof the first lead can be specified highly accurately. Those skilled inthe art will appreciate that additional coordinates may be used todescribe geometric features, e.g., curvature of the leads; or fewercoordinates may be used, e.g., by assuming an azimuth of zero.

FIG. 4 is a flow diagram illustrating a routine 400 that the disclosedtechnology may invoke in various embodiments, e.g., to identify aposition of a second lead relative to a first lead. The routine 400begins at block 402.

At block 403, the routine 400 selects an initial parameter or set ofparameters, e.g., r=0.2 centimeters; z=Φ=θ=0; and R_(j) equal tomeasured or detected one-to-many-contacts impedance values, e.g., foreach contact j. These initial parameters may represent a lead positionbelieved to be most likely; they may be based on previous lead positionand impedance estimates, or measurements; or may be selected randomlyfrom a physically plausible range of parameters. Those skilled in theart will appreciate that a population of parameter sets may be selectedat block 403, each member of the population representing a point in thesolution space, and additional steps may be applied to each member ofthe population to facilitate solution using a population-based algorithmsuch as a genetic algorithm. As an example, the routine may employ amodel to predict expected impedances to each member of the population.

At block 404, the routine 400 employs a model (e.g., a “forward model”300), e.g., to predict the expected impedances based on a set ofparameters supplied to the forward model. Forward modeling techniquescan be used to predict one or more values based on observed parametersby applying or varying one or more modeling functions. The forward modeltakes as parameters r, z, Φ, and θ that can be used to specify theposition of a second lead relative to a first lead. The forward modelalso takes as parameters impedance parameters R_(j), e.g., for eachcontact j of leads 112 and 212. When each lead has eight contacts, j cantake on values 1 through 16 and so there would be sixteen impedancevalues. The impedance parameters can indicate the electrode-tissueimpedance at each contact. The impedance parameters R_(j) may representthe actual electrode-tissue impedance specific to the contact, and canbe used to predict the expected impedance between, for example, eachcontact and a set of other contacts. The impedance parameters R_(j) maybe similar to the predicted, expected impedances and the actual,measured impedances, but they are not necessarily equal to either.

In some embodiments, the impedance between any two contacts j and k isassumed to be the sum of R_(j), R_(k), and a function f(dist_(jk)) ofthe Cartesian distance (dist) between the two contacts j and k. Thetechnology can model impedance between one set of one or more contactsand another set of one or more contacts using well known rules foraddition of resistances. As an example, the impedance between a contactj and a set of contacts k . . . n can be modeled by Equation (1):

R _(total) =R _(j)+((R _(k)+ƒ(dist_(jk)))⁻¹+ . . . +(R_(n)+ƒ(dist_(jn)))⁻¹)⁻¹  (1)

In various embodiments, the technology may employ more complex or lesscomplex models of electrode-tissue interface impedance and tissueimpedance between two electrical contacts. These more complex or lesscomplex models may include characteristics of the electrode-tissueinterface impedance and medium impedance such as linearity,nonlinearity, isotropy, anisotropy, or frequency-dependence, otherstructures affecting impedance such as vertebrae or spinal tissue, ormethods such as finite element modeling for calculation of impedances.Equation 1 and/or the function f(dist_(jk)) can accordingly be adaptedto suit the various embodiments. Quantitative characteristics of thesemodels, such as tissue resistance per unit distance, can also be treatedas input parameters to the model and optimized by routine 400.

The forward model can output a set of simulated (e.g., predicted)impedance measurements for each set of parameters r, z, φ, θ, and R_(j)(e.g., for each contact j). These simulated impedance measurements maybe selected to correspond with actual impedance measurements that havebeen collected; as an example, if an impedance measurement has beencollected between each contact and the set of all other contacts, theforward model can be used to output simulated impedance measurementsbetween each contact and the set of all other contacts.

At block 408, routine 400 computes a fitness value for the selectedparameters. A fitness value is a computation of a similarity between theoutput of the forward model (e.g., the simulated or predicted impedancemeasurements) and the impedances actually detected or measured. Thetechnology in various embodiments employs a log-likelihood computationfor the fitness value. The technology computes the fitness value F of animpedance calculated or predicted by the forward model, R_(jk-calc), inrespect to a measured impedance R_(jk-meas) using equation (2):

$\begin{matrix}{F = {- {\ln \left( \frac{1}{2\left( {1 - L} \right)} \right)}}} & (2)\end{matrix}$

where L is a likelihood computed using equation (3):

$\begin{matrix}{L = {0.5*{{{erf}c}\left( \frac{- {{R_{{jk} - {calc}} - R_{{jk} - {meas}}}}}{\sqrt{2}\sigma} \right)}}} & (3)\end{matrix}$

and where erfc is a complementary error function that is well known inthe field of statistics. In equation (3), σ is the standard deviation ofR_(jk) impedance measurements. The value used for σ may be based on thestandard deviation of actual impedance measurements or may be selected,e.g., by a practitioner according to clinical conditions, thepractitioner's judgment, etc.

Thus, when R_(jk-calc) and R_(jk-meas) are identical, L=0.5 and F=0.Moreover, when R_(jk-calc) and R_(jk-meas) diverge by exactly onestandard deviation σ, L=0.84 and F=−1.148. When L is approximately oreffectively equal to 1 because of round-off error, a minimum value canbe assigned to F.

Overall fitness for a particular set of outputs from the forward modelcan be computed by summing F for each calculated versus measuredimpedance pair. The fitness can be further refined by addingregularization components to favor plausible and parsimonious solutions.As an example, a set of parameters describing a pair of leads positionedat right angles at a great distance from each other may yield the bestfit to measured impedance values, due for example to random noise orother sources of measurement error. However, it may be assumed thatsmall angles of inclination θ are most plausible, all else being equal.An additional term may be added to the fitness computation to expressthe likelihood of the observed deviation from θ=0. In this way, moreunlikely values of r, z, φ, and θ can be made to result in more negativefitness values, and in the example above, more plausible solutions maybe favored even if the fit to measured values is not perfect.

According to testing completed by the applicants, log-likelihood fitnessyields more stable results than some other criteria, e.g., mean-squarederror fitness, Cartesian distance criteria, etc. However, in otherembodiments, the technology may nevertheless employ mean-squared erroror Cartesian distance to evaluate fitness. In some embodiments, thetechnology may also compute fitness against multiple sets of detected ormeasured impedances, detected or measured within a relatively shortduration, e.g., one clinic visit. In these embodiments, summing thefitness values from each computation may yield an overall fitness valuemore robust to measurement noise.

At decision block 410, routine 400 may determine whether an optimalfitness value has been computed. A fitness value may be deemed optimalif, for example, it is approximately or substantially equal to zero, ifit is greater than a predetermined threshold, or if improvement withsuccessive iterations of routine 400 is less than a predeterminedthreshold. If an optimal fitness value has been computed, the routinecontinues at block 412. Otherwise, the routine continues at block 413.

At block 413, routine 400 varies one or more parameters. As an example,the routine 400 may vary r, z, φ, θ, and R_(j). In some embodiments, thetechnology may employ a variant of a simulated metal annealing techniqueto vary the parameters. An initial set of results can be computedstarting from r=0.2 centimeters, z=Φ=θ=0, and R_(j) selected usingmeasured or detected one-to-many-contacts impedance values, e.g., foreach contact j. The “temperature” in the simulated annealing techniquecan then be gradually decreased over several iterations and at eachiteration, a large number of new parameter sets may be generated,randomly perturbing each parameter r, z, φ, θ, and R_(j) by an amountproportional to the temperature, e.g., so that the magnitude of theperturbations decreases with each iteration. Prior to proceeding to asubsequent iteration, a specified number of the most fit (e.g.,parameter sets resulting in a highest F) can be retained. Parameter setsresulting in less fitness may also be retained, with a probabilitydependent on “temperature” and diminishing with each iteration. This canresult in deriving “more fit” values progressively so that a “most fit”set of parameters can be ultimately identified that maximizes thefitness value. In other embodiments, other techniques can be used tovary parameters. The routine 400 then continues at block 404 where a newoutput is generated from the forward model based on the parametersvaried at block 413.

In various embodiments, the technology may use techniques other thanannealing to vary the parameters. These techniques can employ gradients,genetic algorithms, population swarms, etc. One skilled in the art willrecognize that various techniques exist to vary parameters and determinean optimal parameter set.

At block 412, routine 400 can indicate the relative positions of theleads or contacts, e.g., on a display. As an example, the technology mayrender (e.g., display or draw) a graphical depiction of one or bothleads on a display so that the practitioner can visually see how thesecond lead is positioned relative to the first lead, e.g., on a displaydevice. The technology can render changes in position in nearly“real-time” so that the changed positions are evident immediately, e.g.,by repeatedly measuring or detecting new impedance measurements, andrepeatedly using routine 400 to determine relative lead positions foreach new set of impedance measurements.

Those skilled in the art will appreciate that the logic illustrated inFIG. 4 and described above may be altered in a variety of ways. Forexample, the order of the logic may be rearranged, substeps may beperformed in parallel, illustrated logic may be omitted, other logic maybe included, etc.

FIG. 5 is a block diagram illustrating components 500 employed by thedisclosed technology in various embodiments. In various embodiments, thecomponents 500 may be implemented in hardware, software, or acombination of hardware and software. The components 500 include animpedance detector 502, a model calculator 504, a fitness computer 506,a population generator 508, population selector 510, and data storage512, e.g., for storing a population of solutions.

FIG. 6 is a user interface diagram illustrating a user interfaceimplemented by the disclosed technology in various embodiments. Invarious embodiments, a user interface 600 can include a first region 602that illustrates a current position of leads; and a second region 604that illustrates a historical record of lead positions. Variousembodiments can include the first region only, the second region only,or both regions. The user interface 600 can be provided at a personalcomputer, a tablet computer, a small handheld device, etc.

A practitioner may employ the first region 602 to quickly determinerelative present positions of leads, e.g., after insertion, and modifyparameters, e.g., in response to changes illustrated in the userinterface. In various embodiments, region 602 may indicate contactsassociated with the leads and impedances detected thereat.

A practitioner may employ the second region 604, e.g., to identifypositions at which the patient indicated maximum relief from pain;determine the period of time a patient has been treated; to determine afollow-up routine or schedule, etc.

The user interface 600 can be provided at a clinical environment (e.g.,where a patient undergoes treatment), at a home (e.g., in relation to ahome monitor), or elsewhere. In various embodiments, the technology cancorrelate currently observed positions with impedance data changes overtime or pain score (e.g., as reported by the patient) changes over time,e.g., to indicate treatment programs or vertebral locations that mayneed to be re-tested. As an example, the user interface may graphicallyshow regions of the patient's vertebra where treatment has been applied,and results therefrom.

The user interface 600 can be provided in conjunction with implantedpulse generators, trial stimulators, or other devices pertinent to thetechnology disclosed herein. In various embodiments, the user interface600 may enable users (e.g., practitioners) to modify parameters, providealerts (e.g., identify a need for a follow-up visit), record movement ofleads, etc.

From the foregoing, it will be appreciated that specific embodiments ofthe technology have been described herein for purposes of illustration,but that various modifications may be made without deviating from thetechnology. For example, in other embodiments, the technology can beused to locate devices other than spinal cord implants. In still furtherembodiments, these devices and methodologies can be applied toimplantable patient devices other than neural modulators (e.g., otherelements configured for patient implantation, with therapy contacts inat least some cases). The implanting tools described above can haveconfigurations other than a stylet (e.g., a catheter) in otherembodiments. The locator signal emitters and/detectors can beomnidirectional in certain embodiments or can be unidirectional in otherembodiments. In certain embodiments, phase shift and/or phased arraytechniques can be implemented to enhance system efficacy. The signaldelivery system can include one transmission device in certainembodiments, and more than one transmission device in other embodiments.

In various embodiments, the leads may have more or fewer than eightcontacts or two leads. In various embodiments, the leads may havedifferent shapes, e.g., cylindrical, conical, toroidal, etc. Althoughthe position of the contacts are illustrated as being equally spaced,other embodiments may space the contacts in different positions andorientations.

In various embodiments, the forward model may employ finite elements orother techniques, e.g., to more accurately estimate impedances. Althoughthe embodiments described above assume that human tissue is homogeneous,e.g., for prediction of impedances, other forward models may not makethis assumption and may take account of inhomogeneous or anisotropictissue models.

In some embodiments, the technology may take account of the spinalanatomy of the patient. As an example, the forward models that thetechnology may employ may vary depending on the presence or absence oftissue, bone, etc., so that impedances can be predicted more accurately.In some embodiments, parameters describing the position of leads withrespect to these anatomical structures may be included in the forwardmodel and optimization process, yielding a determination of positionwith respect to these structures in addition to a determination ofrelative lead position.

In various embodiments, the technology may include for considerationimpedances measured at various points, e.g., various arbitrary pointsassociated with the system in proximity to the patient.

In various embodiments, the leads may be deformed because of contactwith tissue or bone, repeated use, etc. The technology can adapt tothese deformations, e.g., by varying the forward model to includeparameters relating to the extent of deformation. Because of thesedeformations, electrical impedances may vary.

In various embodiments, the technology may employ techniques designed toreduce or eliminate spurious measurements, e.g., by ignoring measuredimpedances that are outside of expected norms.

Certain aspects of the technology described in the context of particularembodiments may be combined or eliminated in other embodiments. As anexample, of parameters r, z, φ, and θ, the z parameter may be givenadditional or exclusive consideration as compared to the r, φ, or θparameters. In various embodiments, more or fewer parameters may beemployed. As an example, tissue impedance may be specifically varied.Further, while advantages associated with certain embodiments have beendescribed in the context of those embodiments, other embodiments mayalso exhibit such advantages and not all embodiments need necessarilyexhibit such advantages to fall within the scope of the presentdisclosure. Accordingly, the disclosure and associated technology canencompass other embodiments not expressly described or shown herein.

1-18. (canceled)
 19. A system for treating patients, comprising: a processor and memory; a forward model component; a fitness computer component; and an impedance detector component.
 20. The system of claim 19, wherein the forward model component is configured to take as a parameter at least a z-coordinate of a second lead in comparison to a first lead.
 21. The system of claim 20, wherein the forward model component is configured to generate a predicted electrical impedance by predicting an electrical impedance at a contact wherein the electrical impedance is a measure of contact-tissue impedance.
 22. The system of claim 21, wherein the fitness computer component is configured to compute a fitness between the predicted electrical impedance and an actual electrical impedance at the contact measured by the impedance detector component.
 23. The system of claim 22 further comprising a component configured to vary at least the z-coordinate through iterations until a best fit is achieved.
 24. (canceled)
 25. The system of claim 19 wherein the forward model component is configured to identify a position of a second lead of a signal delivery system relative to a first lead of the signal delivery system.
 26. The system of claim 25 wherein the forward model component utilizes a coordinate system to identify position of the second lead relative to the first lead, and wherein said forward model component takes as an input at least one or more of: a z-coordinate parameter of the second lead in comparison to the first lead; an r parameter indicative of either a radial distance between a specified contact on each of the first lead and the second lead or a radial distance between an axis of the coordinate system and a corresponding contact of the second lead; a θ parameter indicative of an inclination of the second lead measured from a zenith located on a z-axis; and a φ parameter indicative of an azimuth of the second lead measured counter-clockwise from a plane formed the first lead and a contact of the second lead.
 27. The system of claim 25 wherein the forward model component utilizes a coordinate system to identify position of the second lead relative to the first lead, and wherein said forward model component takes as inputs each of: a z-coordinate parameter of a second lead in comparison to a first lead; an r parameter indicative of either a radial distance between a specified contact on each of the first lead and the second lead or a radial distance between an axis of the coordinate system and a corresponding contact of the second lead; a θ parameter indicative of an inclination of the second lead measured from a zenith located on a z-axis; and a φ parameter indicative of an azimuth of the second lead measured counter-clockwise from a plane formed the first lead and a contact of the second lead.
 28. The system of claim 25 wherein said forward model component utilizes parameters from at least one of Cartesian coordinates, cylindrical coordinates and spherical coordinates to identify a position of a second lead of a signal delivery system relative to a first lead of the signal delivery system.
 29. The system of claim 25 wherein said forward model component utilizes parameters from Cartesian coordinates, cylindrical coordinates and spherical coordinates to identify a position of a second lead of a signal delivery system relative to a first lead of the signal delivery system.
 30. The system of claim 19 wherein said forward model component comprises a forward model of expected impedance measured between contacts of leads of a signal delivery system.
 31. The system of claim 30 wherein said forward model takes as input various parameters, including at least one or more of geometric parameters, rotational parameters and physical parameters associated with the leads.
 32. The system of claim 30 wherein said fitness computer component is configured to compute a fitness value that compares output of the forward model with actual detected impedance values.
 33. The system of claim 30 wherein said fitness computer component is configured to compute a distance value expressing a difference between an output of the forward model and actual detected impedance values. 